Threaded bar

ABSTRACT

A concrete-reinforcement bar, especially a tension bar, provided with hot-rolled, helically-arranged ribs, which are located on two mutually-facing sides of the periphery of the bar and form parts of a thread. The ribs of the thread extend over roughly a third of the periphery of the bar and are inclined toward one another with their front faces which merge into the smooth bar surface of the core cross section. In order to improve the fatigue strength, and mechanical strength, and to simplify the production of the threaded bars and reduce the required tolerances, the cross-section of the bar has a regular, smooth undulating profile in an axial plane in the region of each rib (2); the wave-shaped tops of the ribs and the valleys between the ribs thus form cavities in the otherwise circular core cross-section. The cavities penetrate into the core cross-section only at localized points.

The invention, according to claim 1, refers to a concrete reinforcementbar that is a threaded bar, and especially a spreader bar.

Threaded bars of that kind have been disclosed in German Pat. No. 17 84630. Said threaded bar has hot-rolled, helically extending ribs situatedon two opposite sides of the bar periphery and forming part of a thread.The ribs each extend in full height only over about one third of the barperiphery and are inclined against each other by their front faces thatconvert into the smooth bar surface of the cross section of the core.The ribs, which are rolled on the circular core cross section, have atrapezoidal cross section with slightly rounded flutes.

Threaded bars of that kind are used, for instance, as loosereinforcement bars in steel concrete structures, or as spread bars inprestressed concrete structures, or also, for instance, in cables ofcable-staid bridges.

An important criterion for the excellence of the threaded bar,especially in case of loose reinforcement bars, is the fatigue strengthwhich depends not only on the quality of the steel but also especiallyon the transition between the ribs and the bar core. Due to thetrapezoidal cross section of the ribs in the already known threaded bar,a notch effect appears in the transition of the ribs to the pin coreswhereby the fatigue strength is reduced. This is a deficiency,especially for the normal loose reinforcement in steel concrete, sinceit gives steel concrete structural parts in which alternative loads havea very large proportion in the total load of the threaded bar and thereinforcement bars are accordingly exposed to strong tensionfluctuations. Especially when the tension fluctuations are positive andnegative load alternations or bending moments in which cases thereinforcements act alternatively as traction and pressurereinforcements, as it substantially occurs in live loads in bridgebeams, the possible stress reversal between ribs and bar cores isreduced. Such positive and negative load alternations are especiallycritical in the field of sleeve joints in which both threaded bars arescrewed to each other. Due to the shape of the threaded ribs and of thecorresponding screw thread in the sleeves, the bars and the sleeves mustbe made with relatively greater tolerance, whereby the sleeve joint hasrelatively great play. In the load alternations involved, thereinforcement on said sleeve joints becomes weakened.

If the threaded bars are used as spread bars, then this deficiency isnot so serious since the threaded bar is always under tension. Thetension oscillations, due to load alternation, more or less make up onlya small fragment of the whole tension.

However, when those threaded bars are used, especially in cables,tension oscillations can appear, depending on the intended use, thatrequire a maximum fatigue strength from the spreader bars also. Bendingmoments are to be especially mentioned in the area of the point ofsupport of the threaded bars.

Problems also arise in the manufacture of the already known threadedbar. Thus, on account of the trapezoidal cross section of the threadribs, the rolls with which the threaded pins are produced can only bemade by hand. Besides, the narrow rounding on the flutes between threadribs and bar cores forbid the use of mechanized cutting tools. Therelatively high thread ribs, with steep sides, are difficult to roll andduring the rolling operation are fully filled with material only whenquite a specific narrow temperature range is maintained. This requireslong experience of the rolling mill personnel and rejected material mustbe expected. Different ways have been sought to reduce said problems inthe rolling operation. One threaded bar resulting therefrom, which alsois used in the practice, has been described in U.S. Pat. No. 4,056,911.

Depressions are dug between the thread ribs in this threaded bar, in thecircular core cross section on both sides of the interface of top andbottom rolls, a longitudinal rib resulting between said depressions intowhich the thread ribs run. But is has been shown that intersectionlongitudinal and transverse ribs are disadvantageous, since the brittlefracture sensitivity is thereby increased and the threaded bar gives ahigher fatigue failure.

However, the shaping of said threaded bar is advantageous since duringthe hot rolling the thread ribs are more easily filled with material,thus reducing the number of rejects.

The invention is based on the problem of indicating a threaded bar ofthe kind in question, which can be easily produced, has a highmechanical strength and fatigue strength, even in the presence of loadalteration, and requires tolerances that can be reduced, especially inthe field of joints such as sleeve joints or attachments points.

Accordingly, the cross section of the thread ribs has a smooth course, adepression respectively being provided between adjacent thread ribswhich smoothly joins up the course of the ribs. These ribs aresuperposed on the otherwise rounded core cross section, the depressionssituated between the ribs meshing in the core cross section. The flatdepressions convert with an almost constant transition into the surfaceof the core cross section. The flat depressions act in the rollingoperation as pressure points by which the thread ribs are more easilyand uniformly completely filled with material. The lowest point of thedepression is in the longitudinal symmetrical plane of the threaded barcontaining the center of the thread ribs. Pairs of opposite depressionsserve as pressure points. The filling of the ribs during the rollingoperation is favored, in addition, by the rounded shape in the area ofthe rear of the thread ribs. Besides, the cross sectional surface of thethreaded bar is practically constant over its length, which is likewiseadvantageous for the rolling operation.

The front sides of the thread ribs are inclined against each other likethe threaded bar disclosed in Pat. No. 17 84 630. This is necessary tomake exit of the threaded bar from the rolls possible following the hotrolling and later to precisely direct the threaded bar.

The regularly smoothly helical course of the ribs and the attacheddepressions in the longitudinal symmetrical plane of the threaded barcutting into the ribs can be composed, for instance, of two circlesegments; a course that substantially follows a sine path is alsopossible.

With such molding of the threaded bar, its fatigue strength and itsmechanical strength also are practically not impaired when compared to abar of circular cross section, since no notches exist in the criticalareas and intersecting transverse and longitudinal ribs are absent.

Due to the depressions, practically no weakening of the cross sectiontakes place since a thread rib on the radially opposite side of thethreaded bar stands opposed to each depression and the thread ribstogether with the bar core contribute to transmitting the tensionalforces.

It is also possible to reduce the required tolerances for sleeve jointswith this molding of the threaded bar so that even threaded barscomposed via sleeve joints in loose reinforcements can be better used.The possibility of reducing the tolerances of the threaded bars andsleeves derives from the fact that the tools needed for producing thethreaded bar especially the rolls, have a longer service life on accountof the lack of flutes and edges; besides, the rolls can be mademechanized. The use of cutter tools is likewise possible.

The invention is explained in more detail in an embodiment withreference to the drawing. In the drawings:

FIG. 1 is a view of a threaded bar according to the invention having apartial longitudinal cross section;

FIG. 2 is a cross section through the threaded bar along II--II in FIG.1;

FIG. 3 is a topview of a threaded bar according to the invention.

In FIG. 1, part of a threaded bar 1 with thread ribs 2 is shown withdepressions 3 therebetween. The thread ribs 2 extend on both sides ofthe threaded bar 1, each substantially over one third of the barperiphery, and front faces 4, inclined against each other, convert intothe otherwise circular bar core 5. The depressions 3 mesh in the barcore 5. The course of the thread ribs 2 and the depressions in alongitudinal section containing the central axis 6 of the threaded bar 1is uniformally helical and is composed, for instance according to FIG.1, of two circular segments K1 and K2 so that no notches or flutingresult during the rolling operation. As shown in FIG. 2, the edges 7 ofthe thread ribs 2, in the transition into the inclined front surfaces 4,are rounded.

In FIG. 3 are shown places having equal radii of the threaded bar, amongothers, the R max line corresponding to the maximum radius of thethreaded bar in the highest area of the thread ribs, the RK linecorresponding to the radius of the bar core and to the lowest point ofthe depression, R min corresponding to the minimal radius of thethreaded bar. A few intermediate lines, each having the same radius, arelikewise marked. The soft and smooth transition between the thread ribsand the depressions and between the depressions and the bar cores can beseen here. The height of the thread ribs, as a rule, is chosen largerthan the depth of the depressions respectively compared to the corecross section.

Accordingly, the cross sectional shapes of the ribs and the depressionsdo not have to be equal. Thus, substantially trapezoidal ribs in crosssection and rounded on the upper edges are possible, as shown in dottedlines in a place in FIG. 1.

The shape of the depressions is also given as an example. It isessential that they mesh locally into the core cross section betweenthread ribs and thus act as pressure points. The ribs are dimensioned ina manner such that during the rolling operation they are entirely filledwith material due to the engagement in the core cross section betweenthe ribs.

It is obvious that the illustration and dimensions in the drawings areby way of example.

I claim:
 1. In a ribbed bar, especially a tensioning bar, havinghot-rolled, helically running ribs situated on two opposite sides of thebar periphery defining an interrupted thread configuration, said ribsextending in full height only over one third of the circumference of thebar and having end surfaces converting tangentially into a smooth sidecore surface of the bar, whereby the periphery of the bar between theend surfaces of the ribs on opposite sides of the bar defines an arc ofa circle, thereby defining a circular core cross-section, theimprovement comprising:said bar having, in a longitudinal section in anaxial plane through the region of said ribs, a regular and smoothsinusoidal undulation (K1, K2) corresponding to the outer periphery ofthe ribs and to the troughs therebetween with smooth transition betweenadjacent troughs and ribs, wherein the troughs between the ribs formdepressions in the circular core cross-section of the bar, the lowestpoint of the depressions being in the longitudinal symmetrical plane ofthe bar containing the center of the ribs, thereby acting as pressureregions during a hot-rolling process to uniformly and completely formthe ribs, wherein a rib is located radially opposite each depression,each said depression converts, on its outer edges near the end surfacesof the ribs, into said core cross-section and the cross sectional areaof the bar is substantially constant over the length of the bar.
 2. Athreaded bar according to claim 1, characterized in that the helicalcourse is composed of two circular segments (2, 3) wherein one circularsegment defines said ribs (2) and the other circular segment definessaid depressions (3).
 3. A threaded bar according to claim 1,characterized in that said helical course follows a sine path.
 4. Athreaded bar according to claim 1, characterized in that said helicalcourse is substantially trapezoidal in the area of said ribs (2').
 5. Athreaded bar according to claim 1, characterized in that the transitionis rounded (at 7) between said thread ribs (2) and their inclined frontfaces (4).